Implantable cardiac pacemakers have been in existence since the latter portion of the 1950s. Continuing advances in lead and pulse generator design have evolved, resulting in a variety of pacing modes and pacing parameters for both single chamber and multiple chamber pacing devices.
Researchers continue to study the electromechanical nature of the heart in search of better ways to diagnose and treat cardiac arrhythmia. For example, researchers intent on understanding the operation of the heart watch for repeatable electrical patterns indicating specific cardiac arrhythmia. If identified, such electrical patterns are useful as a diagnostic tool to determine when specific arrhythmia are about to occur (or are more likely to occur). Electronics in the implantable device may be used to identify such electrical patterns and provide appropriate therapy.
One electrical pattern observed at the onset of ventricular tachyarrhythmia and/or ventricular fibrillation in some cases is the so-called “short-long-short” induction pattern. FIG. 1 illustrates the short-long-short induction sequence. In this example, the first premature ventricular contraction (PVC-1 in FIG. 1) causes the first short interval and the long interval. In this example, the second short interval is caused by a second premature ventricular contraction (PVC-2 in FIG. 1). However, the short-long-short pattern is not observed prior to each episode of ventricular tachyarrhythmia and/or ventricular fibrillation. The short-long-short pattern results in extreme variations in instantaneous ventricular rate.
One attempt at limiting rate variation was offered in U.S. Pat. No. 4,562,841, to Brockway et al., issued Jan. 7, 1986. One device discussed in the '841 patent included a rate smoothing function for preventing the rate interval from changing by more than a predetermined percentage on a cycle-to-cycle basis; and a graceful degradation function for causing the ventricular pacing rate to each day when the atrial rate exceeds a programmed upper rate limit for a predetermined length of time. Among other later works, the Prevent study investigated whether a rate smoothing algorithm decreases the onset of ventricular tachyarrhythmia. The American Journal of Cardiology, vol. 83 (5B), Mar. 11, 1999, pp. 45D–47D. One goal of this rate smoothing algorithm was to prevent abrupt changes in heart rate by pacing and to increase or decrease the pace rate gradually. The rate smoothing algorithm allowed increases or decreases in pacing rate only within a given percentage of the last V—V interval.
However, one limitation of the rate smoothing algorithm is that the rate smoothing algorithm is always executing so the pacing pulses are delivered at a pacing rate which cannot vary outside the given percentage of the last V—V interval. Thus, the rate smoothed device may underpace when cardiac rate would normally increase rapidly and may overpace when cardiac rate would normally decrease rapidly. Additionally, the rate smoothing algorithm views the pacing interval only in the context of the last V—V interval, which is a limited representation of the patient's heart rate. The proposed systems are limited to rate analysis and do not include other sources of information about the patient's condition.
What is needed in the art is a cardiac pacing system for reducing ventricular tachyarrhythmia and ventricular fibrillation which adapts to cardiac demands and which does not does not constrain pacing rate changes to a given percentage of a last V—V interval. The cardiac pacing system should have an accurate representation of a patient's heart rate and should incorporate any other information which may assist in identifying possible ventricular tachyarrhythmia and ventricular fibrillation.